1. Field of the Invention
This invention relates generally to the field of testing transducers, such as accelerometers, and relates in particular to methods and apparatus for in-situ testing of a closed loop transducer, more particularly, an accelerometer used in a seismic data acquisition system. Still more particularly the invention relates to a method for generating a test bitstream for in-situ testing of a closed loop transducer having an Analog-to-Digital Converter which includes a .SIGMA..DELTA. modulator.
2. Description of the Prior Art
It is known to test a sensor by applying a physical input to it. For example, a conventional geophone includes a coil which can move in a magnetic field. Movement of the coil causes a current to flow in the coil. As a result, a voltage is developed across an input impedance. Thus, physical shaking of the geophone produces an output signal, and a test shaker could be used to test and characterize geophones.
The method of physically shaking transducers is not suited to mass manufacturing of certain transducers, such as geophones. Physical shaking is impractical to use in-situ where a geophone is placed in a remote location for recording geophysical signals characteristic of acoustic reflections from earth strata.
Electrical testing techniques for in-situ testing of geophones are known. Such tests apply analog signals, e.g., a test current, to flow in the moving coil of the geophone. A back EMF or voltage is produced across the coil in response. The test signal is the analog current applied to the coil; the measured signal is the back EMF voltage produced across the output impedance of the coil.
Prior methods and apparatus for in-situ electrical testing of a transducer, e.g., a geophone, have required separate test hardware which is located near the transducer for optimum performance. In one prior art system e.g., the I/O System Two data acquisition system, an analog test signal is applied into the field coil of each geophone. Such analog signal is generated in a module including a Digital Look Up Table which produces a word signal of 10 to 20 bits. A Digital to Analog Converter (DAC) and an optional filter for the test analog signal are employed for applying the analog signal to one or more geophones. An alternative module includes a digital Sigma Delta Sigma (.SIGMA..DELTA.) modulator which produces a one bit word, a one bit DAC and an optional filter for the test analog signal applied to one or more geophones.
Another class of transducers exists in the form of micromachined accelerometers with capacitive or piezoresistive sensing elements. For mass production applications such as the automotive industry, a go/no-go self test is normally required. One prior electrical technique has been to input a predetermined current through an on-chip element. Such current causes motion of the seismic mass which is detected as an output. Such prior art method for testing micromachined accelerometers is not a high performance method of testing. The term high performance in this context is the use .SIGMA..DELTA. modulators to produce a pulse density modulated signal. Open loop accelerometers (the most common type) are not easily tested with a high performance electrical drive signal.
Prior physical tests of micromachined accelerometers include a "tumble test" where an accelerometer is placed in the earth's gravitational field, and an output is measured as a function of the accelerometer's orientation in the gravitational field. Alternatively, a physical test of a micromachined accelerometer can take the form of a shaker input and a measured output.
The prior art has provided for testing of Sigma Delta modulators per se. Self tests of such .SIGMA..DELTA. modulators have included providing a digital bitstream to the modulator which is then converted to an analog signal for high performance testing of the A/D converter. A one-bit DAC and an optional filter are provided. Various methods for placement of the test circuitry on the micromachined accelerometer have been devised.
There is a certain class of accelerometers which include a micromachined sensor with a closed loop feedback signal which is generated by a .SIGMA..DELTA. modulator. An example of such accelerometer is disclosed in U.S. Pat. No. 4,922,756 which issued on May 8, 1990 to the assignee of this application. Sigma Delta (.SIGMA..DELTA.) modulation based A/D converters are used as an alternative to conventional A/D converters. Sigma Delta (.SIGMA..DELTA.) modulation uses oversampling and one-bit quantization to achieve high resolution A/D conversion at a lower rate.
Identification of Objects of the Invention
A primary object of the invention is to provide a digital method for in-situ testing of a closed loop transducer with a Sigma Delta (.SIGMA..DELTA.) feedback loop.
Another object of the invention is to provide a sensor based closed loop transducer arrangement which has a digital timing pattern where the feedback forcing time segments are split up: one for normal non-testing force feedback to the sensor and a second for forcing the sensor up or down according to an injected test bit of a test bitstream.
Another object of the invention is to provide a method for generating a test bitstream for testing a high performance closed loop Sigma Delta (.SIGMA..DELTA.) based transducer system.
Another object is to provide a method for repetitively using short-length test patterns to generate a continuous test bitstream for testing a Sigma Delta (.SIGMA..DELTA.) modulator based transducer system.